France Geological Evolution Part I

France Geological Evolution 1

According to Hyperrestaurant, the great orogenic movements of the Hercinian era affected the entire French territory. In the Alps and also in the Pyrenees there are evident traces even of bending of the Carbonic, accompanied by metamorphism and by injections of eruptive magma. A small carboniferous basin has been preserved in the Alps (La Mure), and of others remains are observed in the Eastern Pyrenees. The Hercinian hoof exists deep beneath the folds of the Jura and, during drilling, was found in the region east of Lyon. But it is understood that the ancient structure, although deeply attacked by erosion, which leveled the folds by filing them down to the roots, can be better observed in the Hercinian massifs proper. The Armorican Massif and the Ardennes Massif present the greatest interest, for the series of primary layers that appear there, from Cambric to Carbonic. In Brittany there are: a Precambric schist, often metamorphosed and injected with granite, on which the Siluric appears in discord, formed many times of schists with slates and sandstones mostly transformed into quartzites; the Devonico with schists and sandstones; Carbon with schistose facies. Beautiful fauna of the Silurico and especially of the Devonico have been described for various locations. The Ardennes have a more varied range and limestone is not nearly absent like in Brittany. The works of J.-A. Gosselet have made classic the section of the deep valley of the Meuse, which, starting from the slate shales and the Cambric quartzites, crosses all the planes of the Devonico schistose and limestone, with a very interesting coral facies. In the Vosges massif, erosion has flattened the folds up to the deeply metamorphosed parts. Granites and gneisses dominate you; and the carbonic was conserved only in a tight syncline. The Permic has come to cover the ancient plain massif, with layers of sandstone, which have been crossed or covered here and there by eruptive flows; then, it was in turn attacked, following new ground movements, and covered in transgression by the sandstones of the Triassic. Even in the Central Massif, the Hercian base was profoundly leveled by erosion. The primary or metamorphosed strata are almost entirely missing except in the synclines where deposits and sometimes carboniferous basins have been preserved, as in the narrow synclin that crosses Auvergne from Champagnac to Moulins, and.

What remains of the primary outcrops in France allows us to reconstitute the Hercinian edifice, assuming that the guiding lines of the corrugations at the end of the Primary formed a somewhat acute V in the south and more open in the north. The hinge still exists in the Massif Central; the folds of the southern Armorican Massif are clearly oriented SE.-NW.; while in Cotentin the W.-E direction prevails: in the Vosges the SW.-NE direction predominates, as in the east of the Massif Central.

During the Secondary, most of France participates in the history of the Hercinian area of ​​central Europe, which can be summarized as follows: transgressions and regressions of the seas on the folded base, leveled by erosion, except in some higher areas, often spared or covered only from shallow waters, which give littoral facies, while on other depressed areas, almost constantly submerged, sediments of considerable thickness accumulate, where limestones abound. The elevated areas correspond more or less to the Hercian massifs, now well identified: Vosges, Ardenne, Massiccio Centrale and Massiccio Armoricano; depressed areas to the Parisian and Aquitanian basins.

The beginning of the Triassic is indicated by a general transgression, which left more or less coarse sandstones at every point on the edge of the Hercian massifs. This cover still forms the whole northern part of the Vosges, giving rise to large plateaus covered with forests. The Middle Triassic, with its iridescent marl facies and salt deposits, is also very well developed on that side, forming wet plains in Lorraine, which are covered with ponds in the Saulnois. In Lorraine we also note the Muschelkalk, whose more massive bank produces a small côte in E. di Nancy.
With the Liassic period, a difference begins to appear between the Mediterranean area of ​​south-eastern France, where the waters are deeper, and the Hercian area, where the characteristic deposits of the continental shelf predominate. In Lorraine, in Burgundy and also in Normandy, there are marl and limestone with grifee, which give on the slopes of the valleys, rather deeply carved by erosion, the characteristic convex profiles due to sliding of the soil, and which mostly give rise to very fertile areas. In the Alps there are zoogenic limestones with cephalopods and compact limestones originating from sludge.

In the Middle and Upper Jurassic the diversity between the ancient provinces is accentuated. The Alps are preparing for the formation of a geosyncline, where the often calcareous deposits (especially coral limestones) accumulate in large masses. The Parisian Basin is outlined in the form of a sleeve, between the Armorican Massif, perhaps still joined to Cornwall and at times also to the Massif Central, and a Massif Renano (Vosges, Ardennes). At intervals, especially in the Batonian and in the Rauracian, zoogenic limestones are formed, which will form the cornices of the côtes of the Meuse and the Moselle and the cliffs of the côtesBurgundian, which surround the Morvan. The end of the Jurassic is marked by an emergence, clearly indicated by the continental facies of the Purbekian, while the massive limestones of the Portlandian accumulate in the Alps.

France Geological Evolution 1